Here in Ontario, the process would be to file a claim in small claims court (unless the damages exceed $25,000.00 - in which case you had no business letting a stranger touch the bass!) alleging negligence by the person who damaged the bass. That would cost you about $125.00 to file. But the issues of who was negligent, or whether the wrongdoer has the wherewithal to pay, could be eclipsed by an even greater problem, i.e. finding out just who that jerk who mashed up your bass was, or where they live. Every player in every bar has a name, but it is usually a first name only, or worse yet, a nickname - buzz, lefty, gobby, stevo, whatever, which may bear no relation to their real identity. And if the gig/jam isn't a regular gig for you, or you are on the road and it happens during a local open stage, god save you, since you are the outsider, while stevo, or whomever, is everyone's friend. So you may never actually have a real person to sue, even if you want to come back to wherever-it-is in order to pursue the claim.
When I was on the road, our rule was no one used our gear, and it made us unpopular with some clubs, but we avoided having to worry about these problems and while owners griped they mostly got over it. We did get a bomb threat once, but it was unrelated to the rule, and had more to do with our love of Gram Parsons than anything else.
